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Rock Desertification in the Subtropical
Summary. An overview of world karst indicates that most karst environmental problems, especially rock desertification, occur mainly in tropical and subtropical karst terrains. Population pressure and some malpractices in land use have intensified the processes of rock desertification. In Guizhou Province, the annual rate of rock desertification in karst areas was as high as 933 km2 in the 1980s, although in recent years, many projects to tackle with this fragile environment in south China have been implemented. Measures such as in harnessing underground streams, ecological rehabilitation, and comprehensive development in the poorest karst regions of South China have proved to be successful. However, ecological conditions there vary from place to place as a result of geological, climatic,and topographic differences. Consequently, more research and improved management are necessary for better planning and more effective implementation of rehabilitation measures.
1. Introduction Rock desertification refers to the processes which transform a karst area that was covered by vegetation and soil into a rocky landscape almost devoid of soil and vegetation (Fig.1). It is a serious ecological problem in the subtropical karst regions of South China (Fig.2)and it leads to progressive impoverishment of the local residents. A similar process has occurred in other parts of the world, such as in Southeast Asia and in the Mediterranean basin. The fragility of the karst ecological system in tropical and subtropical regions is the basis of rock desertification, but the vicious sequence of events is triggered by human activities including population pressure, bad land use planning and practices, and air pollution. In recent years, key projects supported by government and international communities to mitigate the problem have been implemented. A series of countermeasures are proving to be successful. However, more research is necessary to underpin the measures being taken. Fig. 2 Areas affected by rock desertification in southwest China
2. The fragile ecological system of the subtropical karst of south China 2.1 The recognition on the fragility The ecologically fragile karst system is characterized by droughts, floods, soil erosion, rock desertification, surface collapse, limited biodiversity, low primary productivity, and poverty of human life. According to the results of IGCP 299 "Geology, Climate, Hydrology and Karst Formation"(1990-1994), the rock desertification in south China is a part of the fragile world karst ecosystem belt that extends from the Mediterranean, through the Middle East and parts of southeast Asia to the central Americas. Karst has been recognised as a fragile environment by the scientific community since at least 1983, when a symposium entitled "Degradation and Rehabilitation of Fragile Environments: Karst Areas and Desert Margins" was held at the 149th Conference of the American Association for the Advancement of Science (AAAS 1983). A similar symposium took place in the same year in Guizhou, China. The warm humid climate of the south China karst with mean annual precipitation values ranging between 1000 and 2000 mm and mean annual temperatures of 15-21oC is very favourable for forest (the natural vegetation) and agriculture. The ecological fragility originates from the particular geological environment associated with karst processes, although the vicious land degradational sequence taking place in recent years is associated with human activities.
2.2 Geological Background and Ecology Karst processes are controlled by the system CO2--H2O--CaCO3 . It is an open triphase disequilibrium system and very sensitive to environmental change(YUAN 1997, this volume). The major geological factors that predispose the south China karst to fragility are : (1) the low soil forming capability of soluble rocks, especially pure carbonates, and their relative shortage in nutritive elements. This results in thin soils with low fertility which readily succumb to rocky desertification, when soil erosion takes place under improper land management. To illustrate this affect, areas of rock desertification in the karst of Guizhou Province expanded at an average rate of 933 km2/a during the 1980s (YANG,1988). (2)The double spatial structure formed after prolonged intensive karstification. This brings about not only loss of surface water and dessication, but also floods when the subsurface drainage capacity is insufficient during storms. Consequently, farmlands in the south China karsts are subjected to alternating droughts and inundation. Small patches of soil accumulate in dolines at the expense of centuries of erosion from surrounding rocky slopes. But vegetation can hardly grow on such soil because of its repeated inundation year after year. Thus reverse "timber lines" can often be seen in depressions in the south China karst, especially in the great expanses of fengcong depressions (Fig.3). Under favourable climatic conditions, forest can grow on the higher bare rocky slopes. But because of the shortage of soil, trees develop very deep root systems to get moisture and nutrition from rock fissures, and even from underground streams.Mosses and algae are usually the pioneer plants that help vegetation establishment in such inhospitable conditions with little or no soil. Through biogenic processes a thin water-bearing layer is formed on the surface of carbonate outcrops(CAO,1995). Its moisture absorption or releasing capacity can be 3-15 times higher than that of bare fresh rock (Fig.4). Under natural conditions, trees establish themselves on thin water bearing layers such as this, and develop into forests, but re-establishment is very difficult once cleared. On the other hand, the species selection that occurs in karst micro-ecological systems characteried by thin soil, shortage of water, and alkaline conditions involves petrophile, xerophile, and calciphile species of the subtropical karst forest, thus limiting potential biodiversity. Table 1 shows the results of a comparison between the Maolan karst forest in Libo county, southern Guizhou, and the Mangshan Forest on granite in southern Hunan Province. Both of them cover about 200,000 ha and are located at a similar latitude and are subject to similar climatic conditions. The table shows that the species of pteridophyte, gymnosperm and the total wood storage in the Maolan Karst Forest are only 20-50% of those in the Mangshan Forest on granite. Table. 1 Species comparison between Maolan Karst Forest, southern Guizhou, and Mangshan Forest on granite southern Hunan (JIANG Z. 1996)
2.3 Some international comparisons An overview of karst environments around the world shows that the double spatial structure of karst is not always disadvantageous to the ecological system. In southeast Asia and Central America, the Cenozoic carbonate rocks have a porosity of 16-44% and consequently a higher moisture bearing capacity than in China. They have also been little affected by neotectonic uplift. Consequently, the negative impact of the double spatial structure is reduced, and rock desertification problems are not as serious as in southern China or the Mediterranean region. In the karst areas of the Russian and Siberian plains, thick glacial or fluvial-glacial sediments are favourable to soil formation, and underground karst permits good drainage. In addition, the alkaline rocks mitigate the impact of acidic bog water. Therefore, karst areas in the boreal ecological system can provide a good basis for sustainable agriculture. The ecological function of vegetation is more than a factor in the
conservation of water resources. In the karst regions of southeast Australia, Eucalyptus
has the function of draining away excess water through evapotranspiration. The clearing
has brought about a water table
3. The triggering factors of rock desertification High population pressure superimposed on and exploiting the fragile karst environment triggers the processes of rock desertification. To meet demand for food in south China, people are obliged to grow maize on slopes steeper than 25o. Soil in fissures between rocks has been tilled, and bare soil after harvests results in severe erosion, that strips slopes of soil in a few years (fig. 5). Bad land use practices such as this leads to rock desertification. Other human activities that aggravate the rock desertification problem include fuel gathering, bush fires, and air pollution. Many karst regions in south China are short of energy resources. To obtain fuel, local residents clear the bush and grass on rocky slopes year after year and so make vegetation regeneration very difficult. At the same time, from many years of practice, farmers consider burnt grass ash to be a good fertilizer for crops. So they set fire to vegetation on slopes in late autumn, and let the ash be washed down onto the farmland in poljes and dolines below. However, such fires often get out of control, burning not only shrubs and grasses, but also killing mosses and algae on rock surfaces under the high temperatures generated. As discussed above, algae and mosses are pioneer plants for karst forest; so their loss makes the recovery of vegetation even more difficult. In some urbanized or industrial areas, air pollution and acid rain have a similar effect in killing pioneer species. In extreme cases, carbonate rock surfaces over broad areas are bleached white following the death of algae and lichen. First hand experience shows that such terribly degraded landscapes do not recover even in 20 years after the responsible damaging factors are removed.
4. Rehabilitation measures In recent decades, a number of key projects funded by government to rehabilitate the rock desert areas in southern China have been implemented and have made progress. Moreover, international agencies including the WFP( World Food Program), World Bank, Australian and New Zealand governments have also made remarkable contributions in tackling these serious ecological problems. Thus over the past decade, there has been a successful accumulation of experience in how to mitigate the water and soil problems of the region. Because of the socio-economic dimensions of the problem, the strategy has gone beyond landscape rehabilitation to include interrelated power and irrigation measures.
Although karst in South China is generally deficient in surface water, its groundwater resources are abundant. According to a hydrogeological survey conducted by the Ministry of Geology, underground streams in the area have a total minimum discharge of 1482 m3/s, i.e, equivalent to another Yellow River. A set of methodologies to exploit these resources have resulted in the implementation of some key projects which have assisted to rehabilitate karstlands. These have included building as underground dams and power stations, doline reservoirs and aqueducts. One of the most successful recent examples is the Wulicong Doline Reservoir in Mengzi County, southern Yunnan, where the storage of 79.49 million m3 of water in a big doline commenced 1995. In combination with a long aqueduct, which delivers water to it from Nanxi River, it will permit the 50 000 ha Mengzi Karst Graben Basin, which is the biggest dry karstland in southern Yunnan, to be well irrigated.
With the support of some key state projects,such as the Forest Protection Zone in the Middle and Upper Reaches of Yangtze River, the Northwest Guizhou Water and Soil Protection Program, and many projects to support designated poverty regions, rehabilitation works in karst rock deserts have been making progress through comprehensive ecological engineering . To facilitate local cooperation, conceptually simple measures are promoted that are conveyed in easily understood expressions . Rehabilitation of karst hills is undertaken by severely limiting human activities on the summits of karst cones, so permitting forest regrowth ("to put a cap on the top of the hill"); by mid-slope contour terrace construction that permits farming yet limits downslope wash of soil ("wear ribbons in the middle of the hill") (fig. 6); and by protective cover around the foot of hills to permit sustainable high agricultural production, sometimes using agricultural membrane - strips of thin plastic sheet (fig. 6) - over arable beds ("cloak the foot of the hills"). In the practice of rehabilitating forest on the top of karst hills, cash crop trees such as lacquer tree (Rhus verniciflua Stockes), mulberry, tung tree(Aleurites fordii Hemsl.), plum, and medicinal herbs, such as honeysuckle and Eucommia ulmoides are planted (ZHOU ZHENGXIAN 1993) according to local conditions, so as to get both economic and ecological benifits. Thus in Longna village of Mashan County, Guangxi, 30 years of closing hillsides to livestock grazing and fuel gathering has facilitated afforestation and has made 25 limestone peaks in an area of just 1 km2 into a source of 200 species of medicinal herbs. But in some of the most extreme areas of rock desertification the only solution is immigration.
With the support of the State Science and Technology Commission, some demonstration projects have been completed that show how rock desert regions can be rehabilitated by comprehensive development. For example, the Luota township in northwest Hunan Province covers about 130 km2. It is on a karst plateau underlain by Permian and Triassic carbonate rocks and perched on a thick sequence of Silurian shale. On the basis of karst research carried out in early 1980s, comprehensive development was implementated, including mountain, water, farmland, and forest management, as well as education and population planning, road construction, mining, and countryside enterprises development. The tops of hills on the plateau are reserved for afforestation by China firs. In lower areas underground streams are exploited for irrigation and hydropower. Such measures have shown it to be possible to reverse rocky desertification and to largely solve the severest poverty problems in relatively few years.
5. Conclusions and prospects These results suggest that continued implementation of present policies in the central and western parts of the country will gradually solve the rock desertification problem in the karst of southern China. But to make more effective use of investment further research and coordination work are necessary. It is now important to make a systematic summary of the accumulated experience gained in tackling karst geoecological problems in South China. Research is required to determine the natural or management reasons why some measures taken are successful, whereas others are not. The karst of South China is a fragile environment controlled by topographical and geological conditions, as well as human use. As a consequence of complicated geological structure, stratigraphic features, and a deeply dissected drainage system, it is divided into numerous small units with different moisture and thermal regimes, and variable biogeochemical conditions. Accordingly, it is quite different from other parts of China. A first imperative is to define and recognise the different karst ecological types of South China, and then to determine the appropriate measures required to mitigate the environmental problems associated with each type. This should run alongside spreading experience from demonstration projects. It is necessary to coordinate the applied geomorphological and geo-ecological work of various projects supported from different sources, and to tackle with care the relationship between the state infrastructure construction in South China Karst and ecological rehabilitation projects in the region.
Acknowledgements This is a contribution to UNESCO/IUGS IGCP 299 and 379 projects, and was supported jointly by the Ministry of Geology and Mineral Resources (project 8502218) and the National Natural Science Foundation of China (project 49070155). We are indebted to W. Back, H. Paloc, P. Milanovic, D. Gillieson, D. Ford, P. Williams, V. Andreytchuk, U. Sauro, P. Bono, S-E. Lauritzen, the late Dr. M. Sweeting, D. Balacz, and J. Quinlan and many other international karst friends for helpful discussions.
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Address of author : Prof.Yuan Daoxian
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